For a transdermal preparation, it is necessary to ensure not only transdermal absorption properties of a drug, but also the storage stability of a preparation. Consequently, various methods for inhibiting degradation of a drug and maintaining a drug stably in a preparation have been investigated.
Such methods include:
(1) if a drug unstable against light is contained, a method of adding a UV absorber in a preparation, or using a backing, a liner, or a packaging bag with light intercepting properties;
(2) if a drug is unstable against oxidation, a method of adding an antioxidant or enclosing an oxygen absorber in a package;
(3) if a drug is unstable against water, a method of selecting an oily preparation ingredient, or adding a water-absorbing ingredient in the preparation, or, according to need, enclosing a desiccant in a packaging bag; and
(4) if a drug is sensitive to acidic or basic conditions, a method of adding a pH regulator to regulate the drug to a pH at which a drug can exist stably. The methods can improve the stability of a drug by eliminating a factor for inducing degradation of the drug.
Meanwhile, the content of a drug may occasionally decrease due to a reaction between the drug and an ingredient present in a preparation. In this case, the most preferable method is to replace the ingredient easy to react with the drug with an ingredient difficult to react. But in this case, preparation performance, such as transdermal absorption properties and preparation physical properties (adhesiveness, skin irritation, etc.) may be changed remarkably, and therefore design of an appropriate formulation may become difficult.
A method of adding a metal chloride, such as magnesium chloride, calcium chloride, and zinc chloride, to a pressure sensitive adhesive has been disclosed (Patent Literatures 1 to 2) as a method for stabilizing migration properties of a drug to the skin.
Further, a transdermal absorption agent, in which a pressure-sensitive adhesive layer containing a predetermined drug, a metal chloride and a pressure-sensitive adhesive, and subjected to a crosslinking treatment, is formed on at least one side of a backing, has been disclosed, and according to the disclosure decrease in the cohesion force of the pressure-sensitive adhesive layer can be prevented (Patent Literature 3).